Jaiku: The Finnish presence service that offers "a way to connect with the people you care about by sharing presence updates with them on the Web and mobile." They offer REST and XML-RPC interfaces to get and set user presence information. One Jaiku mashup is TwitKu that lets you monitor your Twitter and Jaiku accounts on a single page.

Propser: This unique peer-to-peer lending service now offers a SOAP API. What do they do? "The way Prosper works is intuitive to people who have used eBay. Instead of listing and bidding on items, people list and bid on loans using Prosper's online auction platform."

PaySimple: The PaySimple payment authorization service has an API available for custom integration. Access to full documentation is limited to customers and partners. See also the Authorize.Net API.